Histological fixatives are used in the preparation of tissue specimens for microscopic examination. It is common practice to fix tissue specimens prior to subjecting them to sectioning and then microscopic examination. Chemical fixation techniques are widely utilized because they generally cause minimal damage to the cells of the tissue, are convenient to use, and can be employed with a wide variety of different tissue types.
Formaldehyde has been a widely used fixative. It has .been used as the sole or as the principal active agent in fixatives, as well as a component in multi-component fixative compositions. While formaldehyde has many desirable properties in these applications, unfortunately it tends to harden the tissue.
A major concern with the use of formaldehyde is safety. Formaldehyde is considered a carcinogen and histopathology laboratories are subjected to federal regulations regarding its use. The United States Occupational Safety and Health Administration (OSHA) requires the monitoring of employee exposure to formaldehyde. Additionally, the exposure limits, which were originally established in 1987, have been reduced considerably since that time. The time-weighted average, or permissible exposure limit (PEL) for an 8-hour period is 0.75 ppm. An action level of 0.5 ppm and a short-term exposure level of 2.0 ppm have also been established. These restrictions make it increasingly difficult to use formaldehyde as a tissue fixative.
As an alternative, glutaraldehyde, which is not considered a safety hazard, may be used as a histological fixative. Glutaraldehyde has most frequently been used for the fixation of specimens for electron microscopy and preserves ultrastructure better than any of the other aldehydes. However, like formaldehyde, it tends to overharden tissue, so fixation may not be prolonged. Yet, glutaraldehyde penetrates more slowly than formaldehyde. Since it fixes as it penetrates, penetration into the deeper part of the tissue is likely to be impeded, so tissue sections must be thin. Therefore, because of its overhardening and poor penetration properties, glutaraldehyde has not found wide acceptance as a fixative for light microscopy.
Furthermore, glutaraldehyde is not recommended as a fixative if periodic acid-Schiff (PAS) reactions are performed because false-positive results are obtained. Finally, glutaraldehyde irreversibly blocks tissue antigenic determinants.
Therefore, it would be highly desirable to develop a glutaraldehyde-based fixative composition for light microscopy in which the penetration rate is equivalent to or better than that of formaldehyde, so that conventional tissue section thicknesses can be used. It would also be highly desirable to prepare a glutaraldehyde-based fixative composition that will allow PAS reactions to be performed. Furthermore, it would be highly desirable to develop a glutaraldehyde-based fixative composition that does not irreversibly block tissue antigenic determinants. Finally, it would be highly desirable to prepare a glutaraldehyde-based fixative composition that yields tinctorial detail superior to formaldehyde-based fixatives.